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Cluster beams from a CoNd liquid alloy ion source
MICROELECTRONIC ENGINEERING
ELSEVIER
Microelectronic Engineering 30 (1996) 245-248
Cluster beams from a
Co-Nd
liquid alloy ion source
L. Bischoff', J. Teichert', E. Hesse ~, P. D. Prewett b and J.G. Watson b "Research Center Rossendorf Inc. Institute o f Ion Beam Physics and Materials Research P.O. Box 51 01 19 , D - 01314 Dresden, Germany bCentral Microstructure Facility Rutherford Appleton Laboratory Chilton, Didcot, Oxon OX11 0QX; UK
A Cobalt-Neodymium Liquid Alloy Ion Source (LAIS) is investigated with respect to its cluster emission behaviour. Clusters and molecular ions were found. The influence of the source emission current on the cluster emission intensity and also on the cluster mass distribution is studied. The Co-Nd LAIS was used for writing patterning (implantation, sputtering) in a mass-selecting Focused Cluster Beam (FCB) system. Theoretical estimations were carried out concerning the use of the FCB for direct deposition at landing energies of about 100 eV / atom. First experimental results of cobalt FCBs are presented.
1. I N T R O D U C T I O N In recent years cluster beams have gained more and more importance in rnicroelectronic technology or even in tribological investigations. The main application are shallow implantation, high yield sputtering, surface smoothing or cleaning and thin f i l l formation by direct deposition [1]. In order to fabricate new devices by this technology, lateral p a t t e r n i n g m u s t be p e r f o r m e d . This p r o c e s s becomes more and more problematic since the usual methods are connected with a large amount o f contamination. For example, air exposure can be sufficient to disturb a further high quality layer growth. Therefore a contamination-free patterning becomes important (vacuum lithography). Alternatively, the deposition can be selectively performed producing the pattern directly without separate lithography. Such a deposition technology is proposed and will be discussed in this paper. The aim is to investigate the possibilities for deposition of structured conductive layers using focused ionized cluster (droplet) beam deposition. Up to now, focused ion beams have been widely used for ion beam assisted deposition in which organic gas molecules are destroyed by ion
c o l l i s i o n s and c a r b o n or m e t a l l i c f i l m s are deposited. The metallic layers produced by this method contain a large fraction of impurities and possess c o m p a r a t i v e l y h i g h r e s i s t i v i t i e s [2]. Recently, low-energy focused ion beams have been u s e d for d i r e c t d e p o s i t i o n [3,4,5]. S u b - p m r e s o l u t i o n and h i g h - p u r i t y l a y e r s h a v e b e e n obtained. To prevent sputtering and to deposit the ions on the surface, the ions must be decelerated to energies between about 10 eV and 1 keV which is connected with increasing optical aberrations (larger beam spot size and lower current density). The use of clusters instead of low-energy ions for direct deposition has the advantage that the landing energy could be much higher according to the size of the clusters. Like for ion beam direct deposition, the layers produced will not contain any impurities. Furthermore, it may be possible to deposit mixed layers from alloy sources, and ionized cluster beams allow the deposition of films with high quality and with new properties.
2. ION OPTICAL APPROACH For selective direct deposition by a focused
0167-9317/96/$15.00 © 1996 - Elsevier Science B.V. All rights reserved. SSDI 0167-9317(95)00237-5
246
L. Bischoff et al. / Microelectronic Engineering 30 (1996) 245-248
cluster beam, the essential quantity is the mass flow which can be obtained for a given beam spot size at constant kinetic energy per atom. The critical question is whether or not this particle flow is higher than that obtained with a low- energy ion beam. Thereby the result is mainly determined by two competing effects. O n the one hand, the use of clusters allows an increase of the landing energy according to the ehister size, which reduces the optical aberrations connected with the strong deceleration at the target. On the other, the emission efficiency of clusters is lower than that of ions and decreases with increasing cluster size. For comparison, the particle flow of a hypothetical ion optical column has been calculated. The optics is similar to that of the IMSA-100 equipment [6] (four electrode acceleration lens and einzel lens), but extended by an additional deceleration lens in front of the target. For this lens, two designs proposed by Aihara et al. [7] have been used in the calculations. One is with a nearly homogeneous retarding field between a counter electrode and the target (type a in Ref. [7]) and the other is with an additional retarding electrode (RE) in front of the target (type b in Ref. [7]). The beam in the column has an energy of 30 keV. Without deceleration, the axial chromatic aberration coefficient is about 2.5 mm which corresponds to a 100 nm spot size at a current of 500 pA for single-charged ions. Figure 1 shows the results for the mass flow as a function of the cluster size for both retarding lenses. The landing energy per atom is 100 eV and the spot size 500 nm. Curves are calculated assuming a quadratic 0=2) and a cubic (i=3) decrease of the axial angular intensity 7(n) = 111n-i as a function of cluster size n. Furthermore, the figure contains the points which have been calculated using the experimental cluster emission data of the investigated Co-Nd source instead of these analytical functions. It can be seen that for the lens with RE and with large chromatic aberrations at low landing energies the use of clusters always increases the mass flow. The lens without RE has much lower chromatic aberrations. Therefore the mass flow is generally higher and the use of clusters gives no improvement. Because of technical constraints this lens which has the lowest possible aberrations is not usable [7]. But a usable and proper designed lens should have optical
lO9. 10 s •
£0
b without RE
-
-
c
-
~
_
107 .
E
.~0
10~ .
10~ 0 104"
E
lO ~
with RE ;
b, e exp.points c, f ~=-/ln"a I'o I; 2'o cluster size (atoms)
~'5
30
Figure 1: Calculation of the mass flow as a function of cluster size ( 500 n m spot size, landing energy of 100 eV/atom) for two deceleration lenses without and with a retarding electrode (RE) in front of the target (type a and b in Ref [7]).
properties which are between those discussed here. Therefore a higher performance for clusters than for low energy ions can be obtained if the decrease of the cluster angular intensity is not stronger than n -2 which seems to be possible in sources optimized for cluster emission.
3. E X P E R I M E N T A L AND DISCUSSION The Rossendorf focused ion beam IMSA -100 [6] was used to investigate the cluster emission behaviour of a Co-Nd liquid alloy ion source [8]. Up to now the column has no deceleration lens and the experiments have been performend at 35 keV beam energy. Fig. 2 shows the mass spectrum of the emitted heavy particles, i.e. clusters and molecular ions of this source at an emission current of 10 pA, measured with a moveable Faraday cup behind the 150 ~tm beam aperture. The current of the ion/cluster beam decreases with increasing cluster size and is only 2-3 (compared with gallium 3-4 [2]) orders of magnitude lower than that of the single charged ions, as shown in Fig. 3. The Co~+ and Co3+ clusters and for comparison the Co + and Co ++ ions were focused to a Faraday cup with magnetic secondary electron suppression and to a 5 ~tm mesh in the target plane in order to
L. Bischoff et al. / Microelectronic Engineering.30 (1996) 245-248
m/e
5OO0 I I
800
1
(AMU)
1000 I
I
I
247
200
500
I I
I
I
I
I
Co'~
+
CoNd
2
Co4 N d +
3
Co 6
4-
150
4
Co 3 Nd +
5
Co~
6
Co a Nd +
A
•.~
100
C
(1,, L_ L. _'3 u c-
.9
50
1400 AMU
--
+
I
C°4
12345
6
I
0.5
1.0 etec-tric
Piel.d
I
1.5 (kV/cm)
Figure 2. Cluster mass spectra of a Co-Nd liquid alloy ion source at an emission current of 10 ~tA.
determine the beam current and the spot size using the knife edge method. The ion optical column wasoptimized for single charged ions, so that the spot size for the clusters can be still improved. Additionally 20 x 20 ~tm2 squares were structured into a Si target in order to estimate the sputtering yield. The results are summarized in Table 1. Furthermore, the sputtering yield was calculated according to Sigmund's theory [9] using two simple models. First, the cluster binding energy is
assumed to be very high, which means that the cluster acts as one heavy particle with the full energy E of the beam (column A in Table 1). In the second approach, t h e binding energy of the cluster is assumed to be very low. The n particles of the eluster then act individually with the n-th part o f the energy E (column B). The real behaviour may assumed to be between these simple solutions, a n d t h i s is b o r n e o u t b y the experimental results.
L. Bischoff et al, I Microelectronic Engineering 30 (1996) 245-248
248
Table 1 : Properties of the focused ion/cluster beam species
particle flow 109 cobalt / s
spot size (pm)
sputter yield calculatedA
sputter yield calculatedB
sputter yield experiment
Co ++
1.06
0.35
2.56
2.56
2.5 ± 0.2
Co +
2.62
0.12
2.60
2.60
2.6 4- 0.2
Co+2
0.47
0.8
4.04
4.87
4.9 4- 1.0
Co+3
0.04
2-3
4.60
6.87
5.7 + 1.5
Co+4
<0.01
4.94
8.69
Co+s
<0.01
5.03
10.39
ACKNOWLEDGEMENTS The authors want to thank Prof. G.L.R. Mair for helpful discussions. The technical assistance of Mrs. P. Schneider and Mrs. R. Aniol is gratefully acknowledged.
10000'
\\\ \\\
emission current 27.5pA
~\
ta.Z~A •
1000"
REFERENCES
[1] 0
100
[2]
0
1
2
8
4
5
0
?
0
[3]
c l u s t e r size ( a t o m s ) Figure 3: Measured current of cobalt clusters as a function of cluster size.
[4]
[5] 4. S U M M A R Y [6] The d u s t e r emission from a Co-Nd liquid alloy ion source has been investigated. It has been demonstrated that mass-separated beams of light cobalt clusters can be used for patterning in a focused ion beam column. The results show that, f o r direct deposition purposes, a source optimized for cluster emission can deliver a higher mass flow than a low energy ion beam.
[7]
[8] [9]
I. Yamada, Proe. E-MRS, Spring Meeting, Symposium C, May 22 - 26, 1995, Strasbourg, France. P.D. Prewett and G.L.R. Mair: Focused Ion Beams from Liquid M e t a l Ion Sources, Research Studies Press, 1991. S. Nagamaehi, Y. Yamakage, H. Maruno, M Ueda, S. Sugimoto, and M. Asari, Appl. Phys. Lett. 62 (1993) 2143. S. Nagamaehi, Y. Yamakage, M. Ueda, H. Marnno, K. Shinada, Y. Fujiyama, and M. Asari, Appl. Phys. Lett. 65 (1994) 3278. R.G. Woodham and H. Ahmed, J. Vae. Sei. Technol. B12 (1994) 3280. L. Bisehoff, J. Teichert, E. Hesse, D. Panknin and W. Skorupa, J. Vac. Sei. Teehnol. B12(1994) 3523. R. Aihara, H. Kasahara, and H. Sawaragi, M.H. Shearer and W.B. Thompson, J. Vac. Sei. Technol. B7(1989)79. E. Hesse, L. Bischoff and J. Teiehert, J. Phys. D: Appl. Phys. 27 (1994) 427. P. Sigmund, Physl Rev. 184 (1969) 383.
ELSEVIER
Microelectronic Engineering 30 (1996) 245-248
Cluster beams from a
Co-Nd
liquid alloy ion source
L. Bischoff', J. Teichert', E. Hesse ~, P. D. Prewett b and J.G. Watson b "Research Center Rossendorf Inc. Institute o f Ion Beam Physics and Materials Research P.O. Box 51 01 19 , D - 01314 Dresden, Germany bCentral Microstructure Facility Rutherford Appleton Laboratory Chilton, Didcot, Oxon OX11 0QX; UK
A Cobalt-Neodymium Liquid Alloy Ion Source (LAIS) is investigated with respect to its cluster emission behaviour. Clusters and molecular ions were found. The influence of the source emission current on the cluster emission intensity and also on the cluster mass distribution is studied. The Co-Nd LAIS was used for writing patterning (implantation, sputtering) in a mass-selecting Focused Cluster Beam (FCB) system. Theoretical estimations were carried out concerning the use of the FCB for direct deposition at landing energies of about 100 eV / atom. First experimental results of cobalt FCBs are presented.
1. I N T R O D U C T I O N In recent years cluster beams have gained more and more importance in rnicroelectronic technology or even in tribological investigations. The main application are shallow implantation, high yield sputtering, surface smoothing or cleaning and thin f i l l formation by direct deposition [1]. In order to fabricate new devices by this technology, lateral p a t t e r n i n g m u s t be p e r f o r m e d . This p r o c e s s becomes more and more problematic since the usual methods are connected with a large amount o f contamination. For example, air exposure can be sufficient to disturb a further high quality layer growth. Therefore a contamination-free patterning becomes important (vacuum lithography). Alternatively, the deposition can be selectively performed producing the pattern directly without separate lithography. Such a deposition technology is proposed and will be discussed in this paper. The aim is to investigate the possibilities for deposition of structured conductive layers using focused ionized cluster (droplet) beam deposition. Up to now, focused ion beams have been widely used for ion beam assisted deposition in which organic gas molecules are destroyed by ion
c o l l i s i o n s and c a r b o n or m e t a l l i c f i l m s are deposited. The metallic layers produced by this method contain a large fraction of impurities and possess c o m p a r a t i v e l y h i g h r e s i s t i v i t i e s [2]. Recently, low-energy focused ion beams have been u s e d for d i r e c t d e p o s i t i o n [3,4,5]. S u b - p m r e s o l u t i o n and h i g h - p u r i t y l a y e r s h a v e b e e n obtained. To prevent sputtering and to deposit the ions on the surface, the ions must be decelerated to energies between about 10 eV and 1 keV which is connected with increasing optical aberrations (larger beam spot size and lower current density). The use of clusters instead of low-energy ions for direct deposition has the advantage that the landing energy could be much higher according to the size of the clusters. Like for ion beam direct deposition, the layers produced will not contain any impurities. Furthermore, it may be possible to deposit mixed layers from alloy sources, and ionized cluster beams allow the deposition of films with high quality and with new properties.
2. ION OPTICAL APPROACH For selective direct deposition by a focused
0167-9317/96/$15.00 © 1996 - Elsevier Science B.V. All rights reserved. SSDI 0167-9317(95)00237-5
246
L. Bischoff et al. / Microelectronic Engineering 30 (1996) 245-248
cluster beam, the essential quantity is the mass flow which can be obtained for a given beam spot size at constant kinetic energy per atom. The critical question is whether or not this particle flow is higher than that obtained with a low- energy ion beam. Thereby the result is mainly determined by two competing effects. O n the one hand, the use of clusters allows an increase of the landing energy according to the ehister size, which reduces the optical aberrations connected with the strong deceleration at the target. On the other, the emission efficiency of clusters is lower than that of ions and decreases with increasing cluster size. For comparison, the particle flow of a hypothetical ion optical column has been calculated. The optics is similar to that of the IMSA-100 equipment [6] (four electrode acceleration lens and einzel lens), but extended by an additional deceleration lens in front of the target. For this lens, two designs proposed by Aihara et al. [7] have been used in the calculations. One is with a nearly homogeneous retarding field between a counter electrode and the target (type a in Ref. [7]) and the other is with an additional retarding electrode (RE) in front of the target (type b in Ref. [7]). The beam in the column has an energy of 30 keV. Without deceleration, the axial chromatic aberration coefficient is about 2.5 mm which corresponds to a 100 nm spot size at a current of 500 pA for single-charged ions. Figure 1 shows the results for the mass flow as a function of the cluster size for both retarding lenses. The landing energy per atom is 100 eV and the spot size 500 nm. Curves are calculated assuming a quadratic 0=2) and a cubic (i=3) decrease of the axial angular intensity 7(n) = 111n-i as a function of cluster size n. Furthermore, the figure contains the points which have been calculated using the experimental cluster emission data of the investigated Co-Nd source instead of these analytical functions. It can be seen that for the lens with RE and with large chromatic aberrations at low landing energies the use of clusters always increases the mass flow. The lens without RE has much lower chromatic aberrations. Therefore the mass flow is generally higher and the use of clusters gives no improvement. Because of technical constraints this lens which has the lowest possible aberrations is not usable [7]. But a usable and proper designed lens should have optical
lO9. 10 s •
£0
b without RE
-
-
c
-
~
_
107 .
E
.~0
10~ .
10~ 0 104"
E
lO ~
with RE ;
b, e exp.points c, f ~=-/ln"a I'o I; 2'o cluster size (atoms)
~'5
30
Figure 1: Calculation of the mass flow as a function of cluster size ( 500 n m spot size, landing energy of 100 eV/atom) for two deceleration lenses without and with a retarding electrode (RE) in front of the target (type a and b in Ref [7]).
properties which are between those discussed here. Therefore a higher performance for clusters than for low energy ions can be obtained if the decrease of the cluster angular intensity is not stronger than n -2 which seems to be possible in sources optimized for cluster emission.
3. E X P E R I M E N T A L AND DISCUSSION The Rossendorf focused ion beam IMSA -100 [6] was used to investigate the cluster emission behaviour of a Co-Nd liquid alloy ion source [8]. Up to now the column has no deceleration lens and the experiments have been performend at 35 keV beam energy. Fig. 2 shows the mass spectrum of the emitted heavy particles, i.e. clusters and molecular ions of this source at an emission current of 10 pA, measured with a moveable Faraday cup behind the 150 ~tm beam aperture. The current of the ion/cluster beam decreases with increasing cluster size and is only 2-3 (compared with gallium 3-4 [2]) orders of magnitude lower than that of the single charged ions, as shown in Fig. 3. The Co~+ and Co3+ clusters and for comparison the Co + and Co ++ ions were focused to a Faraday cup with magnetic secondary electron suppression and to a 5 ~tm mesh in the target plane in order to
L. Bischoff et al. / Microelectronic Engineering.30 (1996) 245-248
m/e
5OO0 I I
800
1
(AMU)
1000 I
I
I
247
200
500
I I
I
I
I
I
Co'~
+
CoNd
2
Co4 N d +
3
Co 6
4-
150
4
Co 3 Nd +
5
Co~
6
Co a Nd +
A
•.~
100
C
(1,, L_ L. _'3 u c-
.9
50
1400 AMU
--
+
I
C°4
12345
6
I
0.5
1.0 etec-tric
Piel.d
I
1.5 (kV/cm)
Figure 2. Cluster mass spectra of a Co-Nd liquid alloy ion source at an emission current of 10 ~tA.
determine the beam current and the spot size using the knife edge method. The ion optical column wasoptimized for single charged ions, so that the spot size for the clusters can be still improved. Additionally 20 x 20 ~tm2 squares were structured into a Si target in order to estimate the sputtering yield. The results are summarized in Table 1. Furthermore, the sputtering yield was calculated according to Sigmund's theory [9] using two simple models. First, the cluster binding energy is
assumed to be very high, which means that the cluster acts as one heavy particle with the full energy E of the beam (column A in Table 1). In the second approach, t h e binding energy of the cluster is assumed to be very low. The n particles of the eluster then act individually with the n-th part o f the energy E (column B). The real behaviour may assumed to be between these simple solutions, a n d t h i s is b o r n e o u t b y the experimental results.
L. Bischoff et al, I Microelectronic Engineering 30 (1996) 245-248
248
Table 1 : Properties of the focused ion/cluster beam species
particle flow 109 cobalt / s
spot size (pm)
sputter yield calculatedA
sputter yield calculatedB
sputter yield experiment
Co ++
1.06
0.35
2.56
2.56
2.5 ± 0.2
Co +
2.62
0.12
2.60
2.60
2.6 4- 0.2
Co+2
0.47
0.8
4.04
4.87
4.9 4- 1.0
Co+3
0.04
2-3
4.60
6.87
5.7 + 1.5
Co+4
<0.01
4.94
8.69
Co+s
<0.01
5.03
10.39
ACKNOWLEDGEMENTS The authors want to thank Prof. G.L.R. Mair for helpful discussions. The technical assistance of Mrs. P. Schneider and Mrs. R. Aniol is gratefully acknowledged.
10000'
\\\ \\\
emission current 27.5pA
~\
ta.Z~A •
1000"
REFERENCES
[1] 0
100
[2]
0
1
2
8
4
5
0
?
0
[3]
c l u s t e r size ( a t o m s ) Figure 3: Measured current of cobalt clusters as a function of cluster size.
[4]
[5] 4. S U M M A R Y [6] The d u s t e r emission from a Co-Nd liquid alloy ion source has been investigated. It has been demonstrated that mass-separated beams of light cobalt clusters can be used for patterning in a focused ion beam column. The results show that, f o r direct deposition purposes, a source optimized for cluster emission can deliver a higher mass flow than a low energy ion beam.
[7]
[8] [9]
I. Yamada, Proe. E-MRS, Spring Meeting, Symposium C, May 22 - 26, 1995, Strasbourg, France. P.D. Prewett and G.L.R. Mair: Focused Ion Beams from Liquid M e t a l Ion Sources, Research Studies Press, 1991. S. Nagamaehi, Y. Yamakage, H. Maruno, M Ueda, S. Sugimoto, and M. Asari, Appl. Phys. Lett. 62 (1993) 2143. S. Nagamaehi, Y. Yamakage, M. Ueda, H. Marnno, K. Shinada, Y. Fujiyama, and M. Asari, Appl. Phys. Lett. 65 (1994) 3278. R.G. Woodham and H. Ahmed, J. Vae. Sei. Technol. B12 (1994) 3280. L. Bisehoff, J. Teichert, E. Hesse, D. Panknin and W. Skorupa, J. Vac. Sei. Teehnol. B12(1994) 3523. R. Aihara, H. Kasahara, and H. Sawaragi, M.H. Shearer and W.B. Thompson, J. Vac. Sei. Technol. B7(1989)79. E. Hesse, L. Bischoff and J. Teiehert, J. Phys. D: Appl. Phys. 27 (1994) 427. P. Sigmund, Physl Rev. 184 (1969) 383.